John Pearson and his team at China Lake developed technology between the 1940s through the 1970s related to the Pearson Notch, also referred to as a V-notch in which a series of grooves (whether intersecting or not) creates stress raisers and points of fracture initiation within armament bodies, for example, warheads, so that upon detonation of the warhead, the bodies would fragment into multiple smaller projectiles directed in accordance with an engineered plan.
While the engineering aspects of the technology were explored in those 30 years, manufacturing of these warheads by manufacturers has not proven to be as easy over the last sixty years as was initially contemplated.
Specifically, in the 1970s and 1980s fragmenting warheads became more and more popular. Explosives containing RDX or HMX were utilized. The two most popular size control methods employed for fragment size were the use of the Pearson Notch and the opposed notch techniques. Both of these methods provided for the use of solid steel casings. The residual strength after notching could still support the missile flight loads, if required, while still allowing for case expansion before eruption to obtain high fragment velocity.
To provide a Pearson Notch, the inside of the steel cylindrical case is notched in a diamond pattern as illustrated in FIG. 1 (cross section) with a branch to provide a diamond pattern. Even though the notches are typically shallow relative to the thickness of the cylindrical case, they have been found to be effective in initiating a fracture trajectory which travels the outside of the case as the case begins to expand upon detonation of the core explosive in the case. This method has been found to be effective for certain ratios of case thickness and notch spacing. For optimum ratios, it is estimated that 80% of the case mass can be controlled to a desired fragment size. While this is acceptable for some customers, the applicant has discovered that 80% is not satisfactory for many customers.
In order to manufacture these diamond shaped patterns, a broach operation is typically performed in which a broach (which is basically a plug with ascending elevations of teeth arranged in a helical pattern) is pulled by a broaching machine through an inner diameter of a warhead casing with the broach beginning at a first diameter with smaller cutting teeth which proceed in a helical manner to larger size teeth to cut more and more of the internal surface away from the inner surface of the material. The plug (or broach) is pulled through in a first direction which cuts parallel helical grooves. The broach is then reversed in direction, and pulled back through to provide opposing grooves to provide a diamond pattern.
Applicant tried this method of manufacturing with a broach. In the first try, the broach stuck internal to the casing. In the second try, internal bolts of the broach broke and a $150,000 broach was destroyed in the process of attempting to manufacture the warhead casing in accordance with the methodology principally used by Pearson and competitors of this technology for the past 50+ years.
As noted in U.S. Pat. No. 2,948,821, which describes that the attempts made in 1955: “have not been entirely satisfactory and the complete fragmentation control is not exercised and that the formation of such grooves or notches involves added difficulty and expense in fabrication of such bodies.”
In fact, manufacturers have continued to struggle with the precise location and construction of these grooves for over fifty years with the broach technique. One warhead manufacturer has had a specialist work on providing more precise location and machining of the grooves for the past twenty years and still has not come up with a more effective technique than utilizing the broach.
In addition to Pearson grooves, there is an opposed groove method currently in use. The opposed groove method provides for providing narrow tapered or straight grooves cut on the inside and outside of the case directly opposite one another. These grooves are typically cut to a depth of the radius at the bottom of the groove chosen so that the thickness remaining between the grooves provides a required case strength and rigidity while also assuring that the case will break cleanly between opposing grooves upon explosive detonation. The opposed groove technique allows for a wider choice of fragment size, but the case can be weaker compared to the Pearson Notch technique. The opposed groove technique has been found to yield over 90% or more of the case fragmentation into the desired size fragment size as advised by prior art publications.
Additionally, sleeves can be utilized to effectively achieve a similar result by providing one set of parallel helix grooves cut in one direction and another set of parallel helix grooves cut in an opposing direction on inner diameters (i.e., a first set of grooves in an inner sleeve placed within an armament casing having a second set of grooves). Once again, these techniques typically do not yield fragments of the desired size of more than 90%. These limitations appear to the applicant to be directly related to imprecision in an ability to precisely cut the grooves during manufacturing by using the broach.
A broach is typically a plug that is pulled linearly through the bore of the casing and, due to the helical nature of the teeth, twist during the pulling operation to cut the desired helical patterns. There is virtually no control over the twist of the broach as it is pulled. The broach performs its own machining operation due to the angular nature of the helical pattern as it twists through the linear pulling operation. Grooves or notches cut with broaches tend to have rounded edges on the bottom surface (and are not normally provided at sharp angles as would normally be desired by the Pearson Notch technology (or opposed notch technology)). A prior art cross section of notch is shown in FIG. 1 as it is practically done using a broach in many instances. In comparing FIG. 1 to other figures as is performed by the applicant's technology, one will quickly see that sloppiness of manufacturing is believed to contribute to a lack of precision in obtaining a higher percentage of fragmentation than has been achieved with the prior art.
Accordingly, there is a need to more precisely and accurately cut grooves whether providing an opposed groove method on a warhead casing or a Pearson Notch method on a warhead casing and/or using an improved technique.